1. Field of the Invention
This invention concerns switching systems. In particular it relates to the testing of communication connections established through switching systems.
2. Description of the Prior Art
Telephone calls are established in the telephone direct distance dialing network between calling and called telephone stations through a hierarchy of switching systems. Many switching systems in the direct distance dialing network are designated as toll switching offices and are arranged to establish call connections for interconnecting other toll and local switching offices in order that a communication path may be established from a calling telephone station to a called telephone station both of which are served by local switching offices.
A toll switching office establishes call connections to a local switching office on a four-wire basis from an incoming trunk through cross-connected toll switching office equipment to an outgoing trunk. The outgoing trunk is extended by a transmission facility to a local switching office serving the called telephone station. After the call connection has been established the toll switching office equipment outpulses the called telephone station directory number over both a transmit and receive path from the incoming trunk through the toll switching office to the outgoing trunk. The outpulsed called telephone directory number signals appearing on both the transmit and receive path are combined in the outgoing trunk and transmitted over a two-wire trunk facility to the local switching office. The local switching office receives the combined directory number signals and utilizes them to establish a connection with the called telephone station.
It sometimes happens that a pair of leads comprising one path of a four-wire connection through the toll office switching equipment are inadvertently reversed or transposed. When this occurs with a two-wire transmission facility the directory number signals outpulsed by the toll office common control equipment are absorbed by the outgoing trunk instead of being combined. The local switching office does not receive digits of the called telephone station directory number. When this occurs the switching equipment of the local switching office times out after waiting for the directory number signals and the call connection is never completed to the called telephone station.
Telephone toll switching offices are presently arranged to perform several tests on four-wire call connections prior to completing a telephone call. In the first test the toll office common control equipment makes a false cross and ground test on four-wire connections partially established through the telephone switching office. This test checks for the grounding of leads comprising the partial connection and for erroneous short circuits that may appear across the path leads. During a second test the toll office common control equipment conducts a continuity test on each path of the four-wire connection to determine the continuity of leads comprising each path of the call connection. However, neither of these tests detects the presence of transposition of leads in a path through the toll switching office which would prevent the outpulsing of a called telephone directory number and the inconvenience of a lost call to a calling subscriber.
Accordingly, a need exists in the art for an arrangement for testing for path reversals of four-wire connections through a telephone toll switching office which result in the loss of telephone calls and the inconvenience of telephone subscribers. A need also exists for apparatus that responds to the detection of a path reversal of a four-wire call connection by establishing another four-wire call connection through a telephone toll switching office to complete a telephone call which would ordinarily be lost.